1. Field of the Invention
Present invention relates to the manufacturing method of semiconductor device, in particular, present invention relates to a light emitting device comprising a light emitting element (OLED: Organic Light Emitting Device) formed on a plastic substrate. The invention also relates to an OLED module in which an IC""s including a controller, or the like, is mounted on the OLED panel. Note that, in this specification, light emitting device is the generic term for the OLED panel and for the OLED module. Electronic equipment using the light emitting device is also included in the present invention.
2. Description of the Related Art
Recently, technology for forming TFTs (Thin Film Transistor) on a substrate has greatly progressed, and its application to an active matrix display device is actively developed. In particular, TFTs using a polysilicon film have higher field effect mobility (also referred to as mobility) than that of conventional TFTs using an amorphous silicon film, and thus, they are capable of high-speed operation, which makes it possible to control pixels with a driver circuit formed on the substrate having the pixels formed thereon, while, conventionally, such control of pixels is performed by a driver circuit provided outside the substrate.
Since various kinds of circuits and elements are formed on one substrate in such an active matrix type display device, there are various advantages such as reduction in the manufacturing cost, miniaturization of the display device, improvement in yield, and improvement in throughput.
In addition, an active matrix type light emitting device (hereinafter, simply referred to as light emitting device) having OLED as self-luminous elements are actively researched. The light emitting device is also referred to as organic EL displays (OELDs) or organic light emitting diodes (OLEDs).
OLEDs have high visibility because light is self emitted, and are optimal for making a display thin because a backlight like used for an liquid crystal display (LCD) is not required. Along with this, their angle of view has no limits. Therefore, light emitting devices using OLED have thus come under the spotlight as substitute display devices for CRTs and LCDs.
OLEDs have a layer containing an organic light emitting material in which electroluminescence is generated by adding an electric field (hereafter, referred to as an organic light emitting layer), an anode, and a cathode. The organic light emitting layer is formed between the anode and the cathode, and constituted by a single layer or a plurality of layers. There may be a case where these layers include an inorganic compound. There is emission of light in the electroluminescence of the organic light emitting layer in returning to a base state from a singlet excitation state (fluorescence), and in returning to a base state from a triplet excitation state (phosphorescence).
Note that all layers formed between the anode and the cathode are defined as organic light emitting layers in this specification. Specifically, layers such as a light emitting layer, a hole injecting layer, an electron injecting layer, a hole transporting layer, and an electron transporting layer are included as organic light emitting layers. An OLED basically has a structure in which an anode, a light emitting layer, and a cathode are laminated in the stated order. In addition to this structure, the OLED may also have a structure in which an anode, a hole injecting layer, a light emitting layer, and a cathode are laminated in the stated order, or a structure in which layers such as an anode, a hole injecting layer, a light emitting layer, an electron transporting layer, and a cathode are laminated in the stated order.
An active matrix driving system for displaying an image by arranging a TFT every pixel and sequentially writing a video signal is known as one mode of a light emitting device. The TFT is an indispensable element in realization of the active matrix driving system.
The TFT was almost manufactured by using amorphous silicon. However, the TFT using amorphous silicon is low in electric field effect mobility, and cannot be operated at a frequency required to process the video signal. Accordingly, the TFT was used only as a switching element arranged every pixel. A data line driving circuit for outputting the video signal to a data line, and a scanning line driving circuit for outputting a scanning signal to a scanning line were constructed by an IC (driver IC) externally attached and mounted by TAB (Tape Automated Bonding) and COG (Chip on Glass).
However, when pixel density is increased, pixel pitch is narrowed. Accordingly, it is considered that there is a limit in a system for mounting the driver IC. For example, when UXGA (a pixel number of 1200xc3x971600) is supposed, 6000 connecting terminals are required even when the number of connecting terminals is simply estimated in an RGB color system. An increase in the connecting terminal number causes an increase in generating probability of a contact defect. Further, it becomes a factor in which the area (trim area) of a peripheral portion of a pixel section is increased and the compactness of a semiconductor device with this pixel section as a display and the design of an external appearance are damaged. The necessity of the display unit of a driving circuit integral type is clarified from such a background. The number of connecting terminals is greatly reduced and the trim area can be also reduced by integrally forming the pixel section and the scanning line driving circuit and data line driving circuit in the same substrate.
A method for forming the TFT by a polycrystal silicon film is proposed as a means for realizing this. However, even when the TFT was formed by using the polycrystal silicon, its electrical characteristics were finally not equivalent to the characteristics of a MOS transistor formed in a monocrystal silicon substrate. For example, the electric field effect mobility is equal to or smaller than {fraction (1/10)} in comparison with the monocrystal silicon. Further, a problem exists in that an off-electric current is raised by a defect formed in a crystal grain boundary.
In the light emitting device, at least a TFT functioning as a switching element and a TFT for supplying an electric current to an OLED are generally arranged in each pixel. A low off-electric current (Ioff) is required in the TFT functioning as the switching element. In contrast to this, high driving ability (an on-electric current Ion), the prevention of deterioration due to a hot carrier effect and the improvement of reliability are required in the TFT for supplying the electric current to the OLED. Further, high driving ability (the on-electric current Ion), the prevention of deterioration due to the hot carrier effect and the improvement of reliability are also required in the TFT of the data line driving circuit.
A low concentration drain (LDD: Lightly Doped Drain) structure is known as a TFT structure for reducing the off-electric current value. In this structure, an LDD area adding impurity elements thereto at low concentration is arranged between a channel forming area and a source area or a drain area formed by adding impurity elements at high concentration. Further, an LDD structure (hereinafter, called GOLD by abbreviating Gate-drain Overlapped LDD) is known as an effective structure for preventing deterioration of the on-electric current value due to the hot carrier. In this LDD structure, one portion of the LDD area is overlapped with a gate electrode.
The TFT is manufactured by laminating a semiconductor film and an insulating film or an electrically conductive film while these films are etched in a predetermined shape by using a photo mask. However, when the structure of the TFT is optimized to obtain characteristics required in the pixel section and each driving circuit, the number of photo masks is increased so that a manufacturing process becomes complicated and a process number is necessarily increased.
An object of the invention is to provide a technique for improving the characteristics of the TFT and realizing the TFT of an optimum structure for driving conditions of the pixel section and the driving circuit by a small number of photo masks.
To solve the above mentioned problems, a thin film transistor of a light emitting device according to the present invention includes a semiconductor film, a first electrode, and a first insulating film put between the semiconductor film and the first electrode, and also includes a second electrode, and a second insulating film put between the semiconductor film and the second electrode. The first electrode and the second electrode are overlapped with each other, with a channel formation region of the semiconductor film put between them.
In addition, according to the present invention, in case of a TFT in which the decrease of OFF current is regarded more important than the increase of ON current, e.g., a TFT which is formed as a switching element, a constant voltage (common voltage) is applied to the first electrode. This constant voltage is set lower than threshold voltage in case of an n-channel TFT and set higher than threshold voltage in case of a p-channel TFT.
By applying the common voltage to the first electrode, it is possible to suppress threshold irregularity and to suppress OFF current compared with the TFT which includes only one electrode.
Further, according to the present invention, in case of a TFT in which the increase of ON current is regarded more important than the decrease of ON current, e.g., a TFT which is included in the buffer or the like of the driver circuit of the semiconductor device, the same voltage is applied to the first and second electrodes.
In the specification, the driver circuit means a circuit which generates signals for displaying images on a pixel section. A data line driver circuit and a scanning line driver circuit are, therefore, driver circuits.
By applying the same voltage to the first and second electrodes, the spread of a depleted layer is accelerated substantially as in the case of making the semiconductor film thin and it is, therefore, possible to lower the sub-threshold coefficient (S value) of the TFT and to improve the field effect mobility of the TFT. Accordingly, compared with a TFT which includes only one electrode, ON current can be increased. Further, compared with a TFT which includes only one electrode, threshold irregularity can be suppressed. It is thereby possible to decrease driving voltage by using the TFT having this structure in the driver circuit. In addition, since ON current can be increased, the TFT can be made small in size (the channel width thereof can be particularly made small). It is thereby possible to improve the integration density of the TFT.
The circuit diagram of the thin film transistor of the present invention will be described with reference to FIGS. 30A, 30B and 30C. In FIGS. 30A, 30B and 30C, only p-channel TFT is typically shown. An n-channel TFT is opposite in arrow direction to the p-channel TFT. FIG. 30A is a circuit diagram of an ordinary thin film transistor which includes only one electrode. FIG. 30B is a circuit diagram of a thin film transistor according to the present invention wherein two electrodes are provided with a semiconductor film put therebetween, and a constant voltage (ground voltage in this case) is applied to one of the two electrodes. FIG. 30C is a circuit diagram of a thin film transistor according to the present invention wherein two electrodes are provided with a semiconductor film put therebetween and the two electrodes are electrically connected to each other. In this specification, the present invention will be described with reference to the circuit diagrams shown in FIGS. 30A, 30B and 30C.